This invention relates to weighing machines using load sensors such as load cells set on a floor to measure the weight of articles such as food items and industrial machine parts. More particularly, the invention relates to methods of and means for correcting the effects of lower-frequency floor vibrations on weight signals generated and processed by such a weighing machine.
Floor vibrations usually have lower frequencies than the vibrations caused by an object as it is placed on a weighing machine to be weighed. Thus, it may be thought that a filter can be used to eliminate such lower-frequency vibrations from the weight signals, but the cutoff frequency of the filter must be set low, and this makes the filtering time longer, adversely affecting the speed of weight measurements. U.S. Pat. No. 4,624,331 disclosed a weighing machine having not only a scale cell for measuring the weight of an object and outputting a weight signal indicative of the measured weight, but also a dummy cell set on the same floor as the scale cell such that the effects of the floor vibrations can be eliminated from the weight signal by inverting the dummy signal outputted from the dummy cell and adding it to the weight signal from the scale cell. In this manner, the cutoff frequency of the filter can be set higher, and hence the weighing speed is not reduced.
With a prior art weighing machine of this type, however, the scale cell and the dummy cell must be set close to each other because the floor at the positions of the two cells is assumed to be vibrating under the same conditions. If the conditions of vibrations are different at the positions of the cells, corrections cannot be effected accurately by subtracting the dummy signal of the dummy cell from the weight signal. There may even be situations where the error becomes magnified by the "correction". In short, the choice of the position for installing the dummy cell becomes extremely limited, and the degree of freedom in designing the weighing machine is reduced. If there is not much free space in the neighborhood of the scale cell, in particular, it is extremely difficult to find an adequate place for installing the dummy cell for this purpose. It now goes without saying that the difficulty of this kind is magnified in the case of a combinational weighing machine having a plurality of scale cells and calculating combinations of weight signals therefrom to select a particular combination satisfying a predetermined criterion because each of a large number (such as 10 or 14) of scale cells will have to be provided with a dummy cell associated therewith. Moreover, this has the adverse effects of making the machine large and complicated in structure.
Japanese Patent Publication Tokkai 64-32122 disclosed a combinational weighing machine adapted to determine the vibration characteristics of the floor from the average of weight signals outputted from those load sensors not selected in the preceding cycle of combinational calculations and to thereby correct the signals outputted from the selected load sensors. This correction routine is based, however, on the assumption that the floor vibrations are identical at the positions of the plurality of load sensors. If this assumption does not hold, the effects of floor vibrations cannot be eliminated accurately by subtracting such an average value.
Another problem with prior art methods using a scale cell in combination with another cell for detecting the floor vibrations relates to the difference in sensitivity between the two cells. In other words, signal levels from these two cells are usually different even if they are subjected to identical loads. This difference in sensitivity is due not only to the material, shape, size and fabrication conditions of the cells (or sensitivity difference characteristically of the cells) but also to the difference in the load. By prior art correction methods by subtracting the signal indicative of the floor vibrations from the weight signal, only the sensitivity difference characteristically of the cells (hereinafter referred to as the cell sensitivity) was corrected. This was in part because the weight of the target object, which is to be determined, is a part of the load on the scale cell and is basically an unknown. Since the load on the scale cell may be very different from the load on the vibration-detecting cell, however, prior art correction methods ignoring the sensitivity differences due to difference in load (hereinafter referred to as the weight sensitivity) cannot be accurate. Accuracy in measurements is believed to decline as the weight of the object being weighed increases.
Still another problem with prior art methods and means for correcting the effects of floor vibrations is that components of vibration-detecting signals are used constantly to carry out a specified vibration-correcting routine independently of whether the amplitude of floor vibrations is large or not. This is because certain kinds of errors, such as errors due to difference in sensitivity between the scale and dummy cells, quantization errors when analog-to-digital conversion is carried out and weighing errors due to rounding off, should be left uncorrected if the amplitude of floor vibration components is small. In other words, the final error in the weighing will often become larger by an attempt to make corrections if there are no floor vibrations or if the vibrations are so small as to be negligible.
It is therefore an object of the invention to provide a weighing machine capable of providing an accurate weight value independent of whether the amplitude of floor vibrations is large or small.